Advanced technology signalling systems (e.g. communications and electronic warfare (EW) systems) require high performance amplifiers that exhibit high stability, wide bandwidth, low group delay distortion, good phase linearity, low thermal noise, etc. With the introduction of GaAs and other Group III-IV compound semiconductor materials, coupled with submicron fabrication techniques, MESFETS have become especially attractive for millimeter wave, microwave, UHF and VHF amplifiers, and for wideband, linear VCOs for communications and EW applications. However, although a MESFET offers a number of attractive features, enumerated above, its intrinsic input/output impedances produces high input and output VSWRs, causing amplifier instability over a wide frequency range. The high VSWR increases amplifier mismatching power loss, produces a higher noise figure and, for a multistage amplifier, may cause oscillation instability and spurious output signals.
Now, included among conventional approaches for improving the VSWR of an FET are the insertion of circulators upstream and downstream of the MESFET and the use of balanced hybrids. Unfortunately, these proposals inherently decrease the overall gain of the amplifier, while increasing insertion loss, cost, noise figure and physical size. Moreover, they are impractical for millimeter wave amplifier and monolithic microwave integrated circuit (MMIC) applications due to the extreme difficulty in fabricating high performance miniature circulators.
Recent design efforts involving the use of wideband MESFET amplifiers for EW applications have involved the classical feedback approach (employing a resistor and/or inductor) for improving the bandwidth and stability of the amplifier. In early classical feedback schemes, the feedback resistor was used to control the gain and the input and output impedances of a vacuum tube amplifier. (For a more detailed discussion of the basic theory, see F.E. Terman, Radio Engineer's Handbook, McGraw Hill, 1943, pp 395-406, and H.S. Black "Stabilized Feedback Amplifiers" Elect. Eng., Jan. 1934, pp 114-120.) More recent proposals involving the feedback resistor approach include its application to bipolar transistors to achieve wide bandwidth, as detailed in an article by J.B. Coughlin et al, entitled "A Monolithic Silicon Wideband Amplifier from DC to 1 GHz", IEEE Journal of SSC, Dec. 1973, pp 414-419, and using negative feedback to improve the VSWR of a wideband MESFET amplifier, as described in an article by E. Ulrich entitled "Use Negative Feedback to Slash Wideband VSWR", Microwaves, Oct. 1978, pp 66-70. A principal drawback in using such a feedback resistor is the additional thermal noise that is added directly to the amplifier, which reduces system sensitivity and dynamic range. In addition, the finite size of the feedback resistor, connected between the FET gate and drain, often disturbs the FET input and output matching networks, thereby causing a departure from optimum designed values of gain, noise figure, VSWR, etc.